Air separation with a nitrogen refrigeration circuit



May 16, 1967 E. CIMLER ET AL AIR SEPARATION WITH A NITROGEN REFRIGERATION CIRCUIT Filed May 6, 1964 mwozqmxm ommDk a mOmmwmmEOU INVENTOR. EMIL ClMLER RAYMOND HIPPELI EDWARD H. VAN BAUSH ATTORNEY United States l atent ()fi ice 3,3l9,427 patented May 16, 1967 3,319,427 AIR SEPARATEON WITH A NITROGEN REFRIGERATIGN CIRCUIT Emil Cirnler, Port Washington, Raymond Hippeli, Brooklyn, and Edward H. Van Banish, Pearl River, N.Y.,

assignors to Hydrocarbon Research, EEC. New York,

N.Y., a corporation of New Jersey Filed May 6, 1964, Ser. No. 365,240 4 Claims. (Ci. 6213) This invention relates to improvements in air separation plants, and more particularly to improvements in a primary plant which normally produces vaporous products only whereby some liquid products may be simultaneously produced.

In one form of primary plant for the tonnage separation of air, it is common practice to use turbo-expanders to produce the required low temperaure refrigeration to permit the plant to fractionate air at low temperature. It is customary, in such case, to supply sufficient turboexpander capacity to permit the primary plant to be cooled to operating temperature. Such a primary plant, therefore, has inherent refrigeration capacity and could be put in service to produce the necessary refrigeration for some liquid production. However, with the increased gas flow to the turbo-expanders, the oxygen and vapor yield drop oflf considerably at the expense of the small quantity of liquid.

Nevertheless, there are demands for occasional production of liquid either as liquid product or as backup for vapor production wherein the liquid may be produced and stored during less-than-peak loads so that it can be available either to supplement vapor production or to supply the necessary vapor if the plant is shut down on a temporary basis.

While it is apparent that supplementary refrigeration is necessary, the various methods for producing this supplementary refrigeration in view of the added facility and operating cost, tend to establish an excessive cost of the liquid. It is necessary, in order to meet competition, to correlate added facilities costs with the yields of liquid oxygen or nitrogen and minimize the cost of production of the supplemental liquid product.

Our invention concerns the modification of such a primary air liquefaction plant whereby the production of small amounts of liquid products, either oxygen or nitrogen, is accomplished with the minimum cost,

More particularly, our invention relates to an improved process for an air liquefaction plant utilizing reversing exchangers and turbo-expanders whereby small amounts of liquid products can be obtained without materially reducing the overall vapor yields.

Further objects and advantages of our invention will appear from the following description of a preferred form of embodiment of the invention when taken with the attached drawing illustrative thereof, such drawing being a schematic illustration of an air separation plant.

In a plant of this type, air at ambient condition available at 10, is suitably compressed at 11 and passes through aftercooler 12 and knock-out drum 13 from which moisture may be removed. The relatively dry air at above 100 F. and under a pressure in the order of 100 p.s.i.a. then passes through reversing valve 14 and reversing exchangers 16 and 18 after which the cold high pressure air discharges at 20 through check valve 21 and then by line 22 to the bottom of high pressure tower 23. The temperature of the air as it enters the column is just above its dew point.

In the presence of reflux produced, as hereinafter described, a crude oxygen stream containing about 38% oxygen is removed at 24 from the bottom of tower 23 and, in part, passes through heat exchangers 26 and 26a and thence by line 28 through one or more absorbers 30 and then by line 32 and reducing valve 34, through exchanger 42, to low pressure tower 35. A liquid nitrogen stream is also removed in line 50 from high pressure tower 23, is subcooled in exchanger 42 and passes throughreducing valve 52 and thence into the low pressure tower as reflux.

Substantially pure oxygen vapor is separated from the waste nitrogen in the low pressure tower 23. The oxygen is removed at 38, heated in exchanger 26a and reversing exchangers 18 and 16 and discharged as product. The waste nitrogen gas is removed overhead at 48 and is heated in exchanger 26 and then passes through check valve 21 or 21a, the reversing exchangers 18 and 16, and through reversing valve 14 to waste. The exchangers 16- and 18 are well known reversing exchangers which cool and clean feed air from the atmosphere automatically. The air alternates between two passes with the Waste gas which acts as a purge; the principal air impurities being water and carbon dioxide.

A nitrogen vapor stream of high purity, if desired, is withdrawn from the high pressure tower at line 54, flows in line 54a through exchanger 18 wherein it is heated a limited extent and then by line 54b passes to turboexpander 44 to supply the refrigeration for the unit. This is accomplished by discharging the expanded and cold nitrogen vapor in line 6%} through the exchangers 18 and 16 and then to product line 62.

As more particularly described in our copending application Ser. No. 365,279, filed concurrently herewith, by operating the expander 44 at a discharge temperature in the order of 220 F. rather than 275 B, it is possible to recover a greater percentage of oxygen.

A primary air separation plant such as this normally produces a minimum of nitrogen Vapor product and maximum vapor oxygen product of high purities. To this extent, the apparatus and process heretofore described are merely the basis for the present invention.

If it is desired to produce greater ratios of nitrogen to oxygen vapor, or a limited quantity of liquid products, either oxygen, nitrogen or argon, additional refrigeration is required.

Mode 1added expansion of nitrogen If nitrogen vapor product is desired in large quantity, in lieu of oxygen vapor product, a greater amount of nitrogen can be withdrawn in line 54 from the tower 23 and passed through line 54b for expansion in the turboexpander 44 and discharged through line 60 through exchangers 18 and 16, ultimately being discharged at 62. The greater amount of expansion is accomplished by opening valve or increasing the opening of the nozzles in expander '44 which provides the extra refrigeration. The larger volume of cold gas is passed in heat exchange with tower bottoms by means of reboilersubcooler exchanger 26b. This is accomplished by passing bottoms through line 24a through valve 24e, line 24c, exchanger 26b through valve 241 and line 24g to line 28. This crude oxygen then joins with the balance of crude oxygen from line 24.

Liquid nitrogen or oxygen or combinations of both may then be withdrawn at lines 84 and 76 respectively. If desired, a part of the nitrogen reflux in stream may be passed by line 50a through exchanger 50!) in the liquid oxygen line 76 to maintain its low temperature.

Since in this case the amount of vapors condensed in reboiler 77 is reduced due to the withdrawal of more nitrogen in line 54, less oxygen will be generated at a given purity since a definite relationship exists between reboiler duty and oxygen production. In a case such as this, 31% of the air as nitrogen can be expanded and delivered at line 62 as product with a simultaneous production of a limited amount of oxygen vapor. Approximately 1.5 to 2.0% of the air can be delivered also, as a purified liquid product representing a combination of oxygen, nitrogen and argon.

Mode 2sec0ndary circuit1800 p.s.i.g. nitrogen recycle If, however, a high yield of oxygen vapor is required, the above method of producing refrigeration in unsatisfactory, since reboiler duty and expansion of nitrogen vapor are indirectly related and since the oxygen production and reboiler duty are directly related. The direct relation is based on the known requirement that the production of more oxygen vapor in line 38 or more liquid oxygen in line '76 requires more duty in the reboiler 77. The indirect relation is based on the known condition that more expansion of nitrogen from the high pressure tower 23 gives less duty to the reboiler 77. In each case, the duty is reflected in the transfer of heat from the high pressure tower to the liquid in the low pressure tower. It follows then that in order to main tain high reboiler duty some other source of refrigeration must be obtained by means of a secondary circuit.

One form of secondary circuit includes the separation from the nitrogen product line 61 of some nitrogen in line 63 which passes to an external recycle circuit including nitrogen compressor 66, aftercooler 68, and refrigerated (Freon) exchanger 70, thereby delivering such recycle nitrogen in line 72 at a pressure of approximately 1800 p.*s.i.a. and at a temperature of approximately 90 F. This high pressure and low temperature nitrogen may then be expanded in a turbo-expander 74, discharging at a relatively low temperature and at a pressure of approximately 100 p.s.i.a. in line 75 up-stream of the turbo-expander 44 hereinbefore described.

Expander 44, therefore, flows with a capacity equal to nitrogen withdrawn from high pressure tower 23 through lines 54 and 5417 plus the nitrogen recycle in line 63. Such flow thereby providing extra refrigeration transferred to the high pressure tower 23 by way of exchanger 26b. The total nitrogen then flows through line 6% and reversing exchangers 18 and 16 to line 61 and product fraction discharge at 62.

Theamount of nitrogen recycle exceeds by a small amount the liquid products and it is by virtue of this recycle stream which enters the primary plant at low temperature and discharges from the primary plant at high temperature that added refrigeration is imparted to the primary plant.

The principle involved is one which utilizes low temperature refrigeration of the recycle nitrogen by a readjustment of temperatures in the exchangers 18 and 16.

Mode 3sec0ndary circuit-6004100 p.s.i.g. nitrogen recycle Another form of the secondary nitrogen recycle circuit can be accomplished by the expansion of nitrogen in secondary turbo-expander '74 to a lower temperature and lower pressure to correspond with the discharge of turboexpander 44 which then flows to line 46. In this case, the part of the nitrogen in line 63 is appropriately compressed at 66 to a pressure in the range of 600 to 1100 p.s.i.a. and after cooling and sub-cooling, is expanded to a pressure of about 20 p.s.i.a. The low pressure and low temperature nitrogen then flows through valve 175 into line 46. Such an operation shows minimum equipment costs and relatively low power consumption for the production of liquid products.

Mode 4secandary circuit-low pressure nitrogen recycle Another form of secondary refrigerant recycle circuit is accomplished by the compression of the nitrogen recycle in line 63 by compressor 66 to a pressure in the range of 100 to 600* p.s.i.a., such nitrogen being aftercooled in 68 and further cooled with Freon or other refrigerant in exchanger 70. The low temperature, moderate pressure nitrogen then flows in line 72 through turbo-expander 74 discharging through line 75a at a low pressure in the order of 20 p.s.i.a. and a temperature of approximately 200 F.

In this case, the nitrogen flows through supplemental exchanger 26c wherein the recycle nitrogen preheats a portion of a primary expander nitrogen feed gas which flows in line 54c through exchanger 260 to line 54d and joins line 54b just before turbo-expander 44. This is in substitution for the fraction formerly passing through exchanger 18. The recycle nitrogen thereafter flows in line 750 and joins the primary expander discharge gas beyond exchanger 2612 at line 69. The sum total gas then flows through exchangers 18 and 16 to line 61 where the nitrogen product is again removed at 62.

In this form of nitrogen recycle wherein nitrogen is compressed to less than 600 p.s.i.a., a larger quantity of nitrogen is recycled than when the nitrogen is compressed to over 600 p.s.i.a. As a result of this excess flow of nitrogen in exchangers 16 and 18, less nitrogen flow in line 54a is required, such flow being normally heated in its passage in exchanger 18. The lower than normal preheat of the nitrogen to turbo-expander 44 is thus compensated by the preheat in exchanger 260. Thus the secondary circuit in essence, flows nitrogen at approximately 200 F. to the primary plant and merges at ambient temperature imparting refrigeration to the primary plant for the production of liquids. Though the temperature pattern in exchangers 13 and 16 is altered, the exchangers perform the necessary service of cooling air to near the dew point while simultaneously being cleaned of water and CO Mode 5sec0ndary circuit-Joule-Thomson Another form of the secondary circuit is accomplished by the compression of nitrogen recycle in compressor 66 to approximately 3000 p.s.i.a. The nitrogen then passes through aftercooler 63 and further (Freon) cooling in 7 0 to line 72 wherein it is throttled across Joule-Thomson valve 77 to line 54b upstream of the turbo-expander 44;

The advantages of our invention may thus be expressed as follows:

(1) While the production of liquid products by the separation of air can be accomplished in many ways, the preferred methods herein described are simple, low in cost, and efllcient by the use of nitrogen recycle circuits referred to as secondary, when added to primary air separation plants.

(2) The secondary circuit can be added to a primary vapor producing plant at any time after a primary plant is erected for example, with a minimum of simple connections.

(3) The production of liquid will not jeopardize the amount of oxygen that the primary plant is capable of producing.

(4) A primary plant, it designed using reversing exchangers for automatic cooling and clean-up of air, is not jeopardized by the addition of the secondary circuit. Furthermore, the secondary circuit which flows clean nitrogen, free of water and CO requires no clean-up, mechanically or chemically.

(5) Absence of mechanical or chemical clean-up on any and all streams in the system described, permits the continuous operation of the plant without need for deriming.

(6) The amount of liquids that can be produced is in the range of 1.5 to 3% of the air. If, for example, volume units of air are fed to a primary plant, 20 units of oxygen can be produced of which 2 to 3 units are liquid and the remainder as vapor, as shown by the following table.

Vol. of Air N2 Recycle Oxygen V01. N itro. Vol. at 100 p.s.i. KWH, 1,000 approx. s.e.i. Llqfl Vol. P.s.i.a. Vapor Liq. Vapor Liq.

Prima 29.5) 100 20. 5 9. 0 0 Mode l liimary (44 100 13.95 1. 55 31.0 0 46. Mode 2 (29.45) 100 4. 9 1, 800 18. 45 2. O5 11. 0 0 28. 9 100 5. 5 1, 100 18. 45 2. 05 10. 0 0 28. 1 100 4. 1 600 17.01 1. 89 19.0 0 33. 5 100 5. 7 100 17.01 1. 89 19.0 0 30. 9 100 6.3 3, 000 18.45 2.05 11. 0 0 37. 9

l Liq. N can be made in lieu of liq. Oz. 2 10.96 KWH/1,000 s.e.f. of vapor oxygen.

While we have shown and described preferred forms of embodiment of our invention, we are aware that modificat-ions may be made thereto and we therefore desire a broad interpretation of our invention within the scope and spirit of the description herein and of the claims appended hereinafter.

We claim:

1. The method of separating compressed air into its principal constituents which comprises passing said compressed air through a series of reversing exchangers in heat exchange with a relatively cold waste gas to reduce the temperature of the air to substantially its temperature of liquefaction, passing said cold air into a high pressure fractionation zone in the presence of reflux to separate a crude oxygen stream from a nitrogen vapor overhead, cooling said crude oxygen stream and passing said crude oxygen stream to a low pressure fractionation zone in the presence of reflux to produce a high purity oxygen stream and an impure nitrogen vapor overhead, expanding said nitrogen vapor overhead from the high pressure zone in a turbo-expander and passing it through the reversing exchangers counter-current to the incoming air to cool the incoming air, thereby supplying refrigeration for heat exchange with the incoming air, producing a liquid product by recovering some of the nitrogen vapor from the reversing exchanger, recompressing said nitrogen vapor, reexpanding said recompressed nitrogen vapor in a turbo expander doing work, passing said expanded nitrogen vapor in indirect heat exchange with some of the crude oxygen from the high pressure fractionation zone to reduce the temperature in the low pressure fractionation zone, and then passing said expanded nitrogen vapor through the reversing exchangers countercurrent to the compressed air stream, the said nitrogen vapor overhead once removed and subsequently expanded is never returned to the high pressure or low pressure fractionation zones.

2. The method of separating air as claimed in claim 1 wherein the portion of the nitrogen is compressed to approximately 1800 p.s.i.g. and expanded upstream of the expansion step whereby approximately 2 wt. percent of a liquid product is made.

3. The method of separating air as claimed in claim 1 wherein the portion of the nitrogen is compressed to a pressure in the range of 600-1100 p.s.i.g. and is expanded downstream of the expansion step whereby in the order of 1.5 to 2.0 wt. percent of a liquid product is made.

4. The method of separating air as claimed in claim 1 wherein the portion of the nitrogen is compressed to a pressure in the range of -600 p.s.i.g., cooled, and subcooled, and expanded to approximately 20 p.s.ig. in heat exchange with a recycle nitrogen fraction to the expander.

References Cited by the Examiner UNITED STATES PATENTS 2,496,380 2/1950 Crawford 62-30 X 2,83 6,040 5/ 8 Schilling 623 0 X 3,209,548 10/1965 Grunberg et al 62-13 X 3,216,206 11/1965 Kessler 6213 NORMAN YUDKOFF, Primary Examiner. V. W. PRETKA, Assistant Examiner. 

1. THE METHOD OF SEPARATING COMPRESSED AIR INTO ITS PRINCIPAL CONSTITUENTS WHICH COMPRISES PASSING SAID COMPRESSED AIR THROUGH A SERIES OF REVERSING EXCHANGERS IN HEAT EXCHANGE WITH A RELATIVELY COLD WASTE GAS TO REDUCE THE TEMPERATURE OF THE AIR TO SUBSTANTIALLY ITS TEMPERATURE OF LIQUEFACTION, PASSING SAID COLD AIR INTO A HIGH PRESSURE FRACTIONATION ZONE IN THE PRESENCE OF REFLUX TO SEPARAE A CRUDE OXYGEN STREAM FROM A NITROGEN VAPOR OVERHEAD, COOLING SAID CRUDE OXYGEN STREAM AND PASSIN SAID CRUDE OXYGEN STREAM TO A LOW PRESSURE FRACTIONATION ZONE IN THE PRESENCE OF REFLUX TO PRODUCE A HIGH PURITY OXYGEN STREAM AND AN IMPURE NITROGEN VAPOR OVERHEAD, EXPANDING SAID NITROGEN VAPOR OVERHEAD FROM THE HIGH PRESSURE ZONE IN A TURBO-EXPANDER AND PASSING IT THROUGH THE REVERSING EXCHANGERS COUNTER-CURRENT TO THE INCOMING AIR TO COOL THE INCOMING AIR, THEREBY SUPPLYING REFRIGERATION FOR HEAT EXCHANGE WITH THE INCOMING AIR, PRODUCING A LIQUID PRODUCT BY RECOVERING SOME OF THE NITROGEN VAPOR FROM THE REVERSING EXCHANGER, RECOMPRESSING SAID NITROGEN VAPOR, REEXPANDING SAID RECOMPRESSED NITROGEN VAPOR IN A TURBO EXPANDER DOING WORK, PASSING SAID EXPANDED NITROGEN VAPOR IN INDIRECT HEAT EXCHANGE WITH SOME OF THE CRUDE OXYGEN FROM THE HIGH PRESSURE FRACTIONATION ZONE TO REDUCE THE TEMPERATURE IN THE LOW PRESSURE FRACTIONATION ZONE, AND THEN PASSING SAID EXPANDED NITROGEN VAPOR THROUGH THE REVERSING EXCHANGERS COUNTERCURRENT TO THE COMPRESSED AIR STREAM, THE SAID NITROGEN VAPOR OVERHEAD ONCE REMOVED AND SUBSEQUENTLY EXPANDED IS NEVER RETURNED TO THE HIGH PRESSURE OR LOW PRESSURE FRANCTINATION ZONES. 